Hydraulic brake systems are widespread in motor vehicles. So-called “brake-by-wire” brake systems are being used to an increasing extent, in the case of which brake applications triggered by electrical or electronic (externally actuated) control can be carried out. Such brake applications, which are triggered by electronic control units, are utilized, for example, in automatic distance control systems or (emergency) braking assistance functions. Brake systems are also known, which detect the brake actuation only electronically even in the case of a normal brake actuation by the driver and initiate a brake application without any direct mechanical or hydraulic action by the driver.
Known conventional hydraulic vehicle brake systems often comprise wheel brakes (e.g., hydraulically actuated disk brakes), an electrohydraulic brake slip-driving stability control unit (HECU), and a brake actuation device having a vacuum brake booster. The electronic regulator of the electrohydraulic control unit is usually designed according to the so-called “fail silent” principle, i.e., after a fault is detected, the control unit is completely or partially electrically switched into a non-functioning state. An option for the brake system to be actuated by the driver, e.g., without electrical support or the possibility of electrical influence, remains, however.
Known electrohydraulic control units for ABS/ESP motor vehicle brake systems comprise two inlet pressure connections, i.e., one inlet pressure connection for each of the two brake circuits, four wheel-specific outlet pressure connections, a pressure regulating valve arrangement for setting wheel-specific brake pressures at the outlet pressure connections, a low-pressure accumulator for each brake circuit for accommodating pressure medium drained out of the wheel brakes, and a dual-circuit return pump.
In addition, electrohydraulic simulator brake systems are known, in which the brake pedal feel in the normal operating mode is generated by a simulator. In the event of a fault, a direct actuation of the wheel brakes by the driver is still typically possible, however, in a so-called hydraulic fallback mode.
It is to be expected that vehicle brake systems which are suitable for automatically driving vehicles will also come into use in the future. These brake systems must be, in principle, externally actuatable systems or “brake-by-wire” systems. This means that a braking demand can be requested via electronic or electrical control signals and can be implemented by the system without any action by the driver. In this case, a sufficiently high level of availability of the brake system or of the externally actuatable brake function must be ensured, for safety reasons.
For example, brake systems are conceivable, in principle, in which two complete brake systems operate in parallel using different brakes, e.g., brake calipers, on one or more brake disks of a motor vehicle. This parallel design is highly complex, however, and is therefore cost-intensive, and requires a multiplicity of special new designs of the individual brake system components, which makes such a system susceptible to error as compared to available mass-produced products.
A shutoff of the brake system after an initial fault of any kind is not possible for highly automatic or highly automated driving (HAD). In the event of failure of an electronic control unit assigned to the autonomous driving, it should be possible to continue providing a functional, autonomous braking function and a steering function, at least for a predefined time period (e.g., a few seconds), since it must not be assumed that the driver can resume guiding the vehicle immediately after a failure of the control unit, but rather that the driver must first be prompted to take up the task of driving again and must become oriented thereto.
A brake system for motor vehicles is known from WO 2012/028521 A1, which is incorporated by reference, which system is provided with an electrically controllable pressure generating device, a pressure regulating valve arrangement for regulating and/or controlling the wheel brake pressures induced at the individual wheel brakes, and a first electronic control and regulating unit, which controls or regulates the pressure generating device and the pressure regulating valve arrangement, wherein an electrically controllable auxiliary pressure generating device is provided with a second electronic control and regulating unit, by means of which the master brake cylinder can be actuated. In the event of a failure of the first electronic control and regulating unit, however, it is no longer possible to set wheel-specific brake pressures at the wheel brakes by means of the pressure regulating valve arrangement. The brake system is therefore less suitable for highly automated or autonomously driving motor vehicles.